The present invention relates to a method and apparatus for determining the bond stress-slip characteristics of filaments, wires, rods and the like in composite materials such as concrete.
Critical Navy reinforced concrete structures, such as missile test cells and graying drydocks, are designed to withstand large deformations under severe blast and strong-motion earthquake loads.
In these and many other reinforced concrete structures subjected to large deformations in the nonlinear range, one mode of failure is tensile cracking of the concrete and debonding of the reinforcement. The development of design criteria for these structures requires the evaluation of their response in the nonlinear range where severe deterioration of the steel concrete interface takes place. Accurate modeling of the nonlinear response obviously requires an accurate representation of the interface behavior for large deformations.
Where it is important to predict failure or severe damage, proper representation of the bond between the concrete and the reinforcing bar (rebar) is crucial.
The mechanism of bond includes three main components: chemical adhesion, friction, and mechanical interlock between bar ribs and concrete. Initially, for very small values of bond stress of up to 200 psi, chemical adhesion is the main resisting mechanism. If the bond stress is increased, chemical adhesion is destroyed and replaced by the wedging action of the ribs. This wedging action originates crushing in front of the ribs. Once enough crushing has occurred, a wedge of compacted powder forms in front of the rib, with a low angle of incidence (around 30 to 40 degrees), which then produces wedging, inclined transverse cracks, and longitudinal cracks.
If inadequate confinement is provided in testing, bond failure occurs as soon as the cracks spread through the concrete cover of the bar and the test results do not accurately predict a real life scenario. With proper confinement, the bond stress reaches a maximum before decreasing as the concrete between ribs fails and a frictional type of behavior ensues.
Existing bond stress-slip relationships typically ignore the effects of radial stress and deformation. Accordingly, these relationships are strongly dependent on the particular specimen configuration used and vary widely as shown in FIG. 1.
Recent studies have underlined the importance of providing a proper confining pressure to the test specimens.
For example, Untrauer and Henry pulled #6 and #9 grade 60 bars from 6-inch (152-mm) cube specimens subjected to lateral pressure on two opposite faces of up to 2370 psi (16.3 MPa). Bond strength increased with the square root of the pressure.
Robins and Standish pulled 8- and 12-mm (0.31- and 0.47-in) bars from 100-mm (4-in) cubes laterally loaded on two opposite faces. The pull-out load increased more than 100 percent for lateral pressures of about 10 N/mm.sub.2 (4060 psi) did not increase the failure loads. Robins and Austin used the same setup on lightweight aggregate concrete specimens. Again, increases over 100 percent in pull-out load were observed for lateral pressures from 0 to 24 N/mm.sub.2 (3480 psi), with greater increases taking place at low lateral pressures. Similar observations were reported by Navaratnarajah and Speare.
Eligehausen et. al. tested 125 pull-out specimens consisting of a grade 60 bar with a short length (5d, d=bar diameter) embedded in a 305-mm (12-inch) by 7d by 15d reinforced concrete specimen. An increase in unidirectional confinement from 0 to 13.1 MPa (1900 psi) yielded a 25 percent increase in maximum bond resistance. Confinement provided by the transverse steel across the crack plane was not evaluated.
Gambarova et. al. pulled 18-mm (0.70-in.) bars embedded in a longitudinally cracked concrete specimen. External confinement perpendicular to the cracking plane allowed control of the crack opening, which was kept constant during each test. Bond was observed to increase with increasing confinement, i.e., with decreasing crack opening, by up to 40 percent.
Using a setup similar to Gambarova et. al., Modena et. al. studied the effects of constant confining pressure. A 16-mm diameter bar with lugs at 45 degrees was used. During the tests the slip was increased up to 5 mm (0.2 in.), which is about 1/2 of the lug spacing. They reported an increase in bond strength from about 3 to 8.5 MPa (0.4 to 1.2 ksi) for a confining pressure varying from 1.8 to 8.6 MPa (0.26 to 1.25 ksi). They also reported an increase in the crack opening up to a limit value which decreases with the confining stress.
Braam reports work by Vos and Schmidt-Thro for specimens under both radial and lateral pressure, respectively. However Vos' work was a numerical study only. Finite element calculations by Vos indicated a linear increase of bond stress with axisymmetric radial pressures varying from 2 MPa (290 psi) in tension to 22.5 MPa (3260 psi) in compression. Schmidt-Thro pulled 16-mm (0.63-in.) diameter deformed bars embedded 48 mm (1.9 in. ) in an eccentric concrete specimen with a 25l-mm (1-in.) cover. Tests with constant lateral pressure up to 20 MPa (2900 psi) showed gains in bond strength of up to 150 percent with the most pronounced increases occurring at the lower lateral pressures.
In the preceeding studies, confinement was applied uniaxially and not biaxially as in the present invention. The application of only a uniaxial confining pressure requires a more complex model to evaluate the confining pressures at the concrete-rebar interface. As a result, the evaluated pressures are more difficult to determine accurately and the bond stress-slip relationships derived therefrom are accordingly less reliable and useful.
Dorr subjected 16-mm (0.63-in.) deformed bars embedded in a 150-mm (6-inch) diameter cylindrical concrete specimen to tension (i.e. pulled from both ends). The specimens were subjected to confining pressures of up to 15 MN/mm.sup.2 (2175 psi). The confining pressures were established by placing the concrete specimen in an oil filled confining ring and pressurizing the oil bath to exert force on the specimen. It was found that bond stresses could be incremented up to 50 percent. Dorr attributed the large scatter in extant bond stress results to the variations in test specimens used. Unfortunately, Dorr could not develop any shear at the concrete-oil interface. In addition, the use of an end plate introduces unknown confinements.
Hungspreug conducted an extensive review of confinement effects on bond. He found that increasing cover and transverse reinforcement, both of which would increase the confinement on the bar, are generally accepted as increasing bond strength. He also points to an increase of bond with concrete tensile strength (or with the square root of the compressive strength). Empirical relationships have been derived showing the increase in bond strength with increasing concrete strength, increase bar cover, and increasing stirrup area. Hungspreug carried out pull-out tests on cylindrical specimens with constant radial confining pressure. He found a linear pressure up a confinement of 400 psi (2.8 MPa ) at the bar surface.
Unfortunately, Hungspreug used a rubber hose between the concrete specimen and the confining ring which could not transfer shear stresses when pulled on one end. In addition, an end plate was necessary to retain the specimen when the rebar was pulled which introduced unknown confinement stresses. Finally, for confinements above about 400 psi, increases in maximum bond force appeared to have been inhibited by severe radial cracking.
It can thus be seen that a test method and apparatus is needed for determining the bond stress-slip relationship that accurately represents real life scenarios by accurately including the variable of confining pressure in the testing.